What is the meaning or the point of the poem "I heard a fly buzz when I died"

The poem shows the duality of death, that death is most glorious and inglorious, that it is a physical means to a spiritual end, and it is associated with both Christ-like and carrion imagery.

Here's the poem:

I heard a fly buzz when I died;
The stillness round my form
Was like the stillness in the air
Between the heaves of storm.

The eyes beside had wrung them dry,
And breaths were gathering sure
For that last onset, when the king
Be witnessed in his power.

I willed my keepsakes, signed away
What portion of me I
Could make assignable,-and then
There interposed a fly,

With blue, uncertain, stumbling buzz,
Between the light and me;
And then the windows failed, and then
I could not see to see.

The fly is the dominate image in the poem.  It appears in three of the four stanzas.  The only stanza it doesn't appear in is the second, where there is Christ-like imagery "the king."  The key phrase is "last onset," an oxymoron: how can it be last and first?  Well, Christ said he was the alpha and the omega, the beginning and the end.  Is death not the last of the physical world and the onset of the spiritual?

But how is Christ connected with the fly?  The speaker wills away her material possessions, but what happens to her body?  Is it not a feast for the fly, a carrion?  The speaker seems to lament this sticking point.  The speaker expects to see Christ the King after death, but s/he only gets a fly.  This is a spiritual letdown, to be sure.

I always teach this poem with Faulkner's As I Lay Dying, as both have dead women speaking.  Whereas Dickinson is a Christian, Faulkner professes no theology in his black comedy.  Indeed Dickinson's speaker sounds a lot like the nihilistic Addie Bundren in the poem.  So says Darl:

"The quilt is drawn up to her chin, hot as it is, with only her two hands and her face outside. She is propped on the pillow, with her head raised so she can see out the window, and we can hear him every time he takes up the adze or the saw. If we were deaf we could almost watch her face and hear him, see him. Her face is wasted away so that the bones draw just under the skin in white lines. Her eyes are like two candles when you watch them gutter down into the sockets of iron candlesticks. But the eternal and the everlasting salvation and grace is not upon her."